This invention relates generally to atmospheric gas burners and in particular to improvements in such burners to reduce the likelihood of lifting, yellow-tipping, flashback and excessive carbon monoxide (CO) emissions.
It is known, in prior gas burner systems having only a conventional mixing tube, that lifting of flames in a gas burner can cause reduced performance and, if extreme, increased CO emissions. This well-known lifting effect results when too much primary air is being entrained into the burner and usually occurs during the initial operation of the burner when the burner is cold. Equally important, lifting flames are noisy and create the perception of a serious burner problem, which may result in lost sales and expensive, unnecessary service calls. Furthermore, burners which experience only marginal lifting in one area, may experience severe lifting in another part of the country because of natural gas variations. As a result, lifting is a serious concern in gas range burner design.
The lifting problem has defied a solution for many years, because any design change which reduces lifting, encourages flashback. Essentially, lifting occurs when the flame speed is significantly lower than the port velocity of the air/gas mixture and the flowing mixture blows the flame away. Flashback is the opposite instability; the flame speed is significantly higher than the port velocity, and the flame burns back into the burner.
The problem encountered in conventional gas burner designs is compounded because flame speed and port velocity of the gas change significantly as the burner temperature increases after ignition, even if the firing rate of the burner is held constant. In FIG. 3, which represents data derived using a conventional atmospheric gas burner of the type commonly used as surface burners in domestic gas ranges, T.sub.port equals the temperature of the air/gas mixture at the port; U.sub.p equals the velocity of the air/gas mixture through the port; and PA equals the primary aeration as a percent of stoichiometric. Immediately after ignition, the burner associated with the data in FIG. 3 lifted. As the burner heats up, the port velocity increases because the drop in fluid density with heating more than offsets the decrease in entrained primary air. However, the flame speed also increases with port temperature and increases more rapidly than the port velocity. Consequently, the flame stopped lifting, typically, about 45 seconds after ignition.
Two apparent solutions to the lifting problem are: (1) increase the port area, thereby reducing the port velocity; or (2) uniformly reduce the amount of entrained air by increasing the burner loss coefficient. Neither of these is acceptable because the first approach will result in flashback when the firing rate of the hot burner is reduced and the second will cause yellow-tipping (ends of the flame turn from blue to yellow which indicates that the flame is not getting enough air) and high CO emissions in the hot burner, especially for lower firing rates.
Consequently, a more advantageous gas burner system would be presented if excess primary air were avoided at ignition, undesirable reductions in PA at lower firing rates were inhibited and such amounts of lifting, flashback, yellow-tipping or high CO emission were reduced.
It is apparent from the above, that there exists a need in the art for a gas burner system which is efficient through simplicity of parts and uniqueness of structure, and which at least equals the safety characteristics of known gas burner systems, but which at the same time substantially increases performance. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.